In textile yarn processing machines, such as two-for-one twisters, yarn is pulled from a hollow supply package carried by a stationary carrier mechanism in each of a plurality of spindle assemblies and passed through the hollow center of the supply package by directing the yarn into the upper end of a yarn inlet tube and through a yarn passageway in the stationary carrier mechanism and through an adjoining yarn passageway in a rotor mechanism and then radially out of the rotor mechanism. Upon leaving the rotor mechanism the yarn is guided upward forming a balloon which rotates around the supply package, and thence to a take-up package. Originally, threading of the yarn through the yarn passageways provided in the carrier mechanism and the rotor mechanism of such textile yarn processing machines was accomplished manually. This manual threading operation was extremely time consuming and cumbersome and reduced the efficiency of the machine operation. More recently, pneumatically operated threading mechanisms have been provided for automatically threading the yarn through the yarn passageways of the carrier and rotor mechanisms. For example, U.S. Pat. No. 3,975,893 issued Aug. 24, 1976 and assigned to the assignee of the present invention, discloses a threading mechanism wherein an injector nozzle is provided in the stationary carrier mechanism of the spindle assembly for creating a flow of air in the yarn passageways of the carrier mechanism and rotor mechanism when compressed air is supplied thereto.
In order for the pneumatic threading mechanism to thread a yarn through the yarn passageways of the spindle assembly, it is necessary to provide a clear path for threading the yarn through the yarn passageways by opening or releasing a yarn brake mechanism which is usually provided within the spindle assembly adjoining the yarn inlet tube. This has been heretofore accomplished in various ways. For example, in the case of a yarn brake mechanism employing a capsule-shaped braking cartridge, mechanisms have been provided for disengaging the braking cartridge when the yarn inlet tube is manually depressed. As described in German Auslegeschrift No. 2,543,018, for example, depressing the yarn inlet tube during the thread-up operation moves a bridging tube laterally into position to assist in guiding the yarn through the brake housing, with the bridging tube also serving to laterally displace the capsule-shaped braking cartridge. German Offenlegungschrift No. 2,309,578 discloses a somewhat similar arrangement wherein a capsule-shaped braking cartridge is displaced laterally by depressing the yarn inlet tube, with the braking cartridge being held in the laterally displaced position out of the path of yarn travel by a magnet. In both of the arrangements just described, the yarn brake mechanism must be disengaged or released manually by depressing the yarn inlet tube before a thread-up operation can be carried out, which is undesirable when automatic threading mechanisms are used.
In U.S. Pat. No. 3,945,184 issued Mar. 23, 1976, and assigned to the assignee of the present invention, there is disclosed a yarn brake mechanism for a two-for-one twisting spindle assembly wherein the yarn passageway extending through the spindle assembly is locally widened to form a cavity, the upper end of which is in the form of a flat oblique braking surface extending around the yarn inlet opening of the yarn passageway into the cavity. The braking element of this yarn brake mechanism comprises a braking plate made from a ferromagnetic material and which is positioned covering the yarn inlet opening into the cavity. An adjustable permanent magnet is provided for biasing the brake plate toward the braking surface for applying a desired variable tension to a yarn entering the cavity through the yarn inlet opening. In order to pneumatically thread a yarn past a yarn brake mechanism of this type, means were previously provided on the outside of the spindle assembly whereby an operator could manually cause the braking plate to be moved away from the braking surface during a thread-up operation. In order to avoid the need for manual release of the brake mechanism by the operator, the above-mentioned U.S. Pat. No. 3,945,184 has proposed that the stationary braking surface be provided with a bypass channel which adjoins the yarn passageway and through which the yarn is passed during the pneumatic threading operation without the necessity of moving the braking plate off the braking surface. During the subsequent start-up of the spindle assembly the yarn is tensioned and as a result is reintroduced from the bypass channel into position between the braking surface and the braking plate.